The names of the three Hitchhiking Ghosts (Ezra, Phineas and Gus) originally came from cast members and were so popular in the Mansion's fandom that Disney eventually adopted them as official ones.
A story circulated that the Bride's ring was embedded in the exterior exit path of Walt Disney World's Haunted Mansion. It was actually the remains of a crowd-control stanchion that had been cut down. It was removed during 2007's Re-Haunting. In 2011 an "official" Bride's ring was embedded in the path.
The "Aging Man" in the foyer before the stretching portrait room is accepted to be the mortal form of the Master Gracey character seen in the graveyard and on a bell in the Servants Quarters hallway.
In the Phantom Manor backstory, many speculated the Phantom was Henry Ravenswood. This was made officially canon with the 2019 Re-Haunting.
The Bus Came Back: There have been reports that if you ask a cast member or staffer where the Hatbox Ghost has been all these years, they'll say, "He was always in the attic, you just couldn't see him, but he could see you!"
Creator Backlash: Marc Davis, one of the main Imagineers behind the original attraction, was very displeased with Phantom Manor, particularly the derelict look of the building, saying, "Walt would never approve of it." He is also recorded to be less than satisfied with the Mansion, citing the changes to his ideas and "too many cooks".
The Danza: Madame Leota is named for Leota Toombs, the Imagineer who provides the character's face. Her daughter has taken on the face role for Haunted Mansion Holiday.
She then provided the voice and face for Little Leota.
Executive Meddling: According to one of the engineers, the engineers had wanted to bring back the Hatbox Ghost since at least 1999 (then, they were celebrating the ride's 30th anniversary), but everytime they tried to pitch the idea to Disneyland execs, they were immediately shot down with the reasoning, "the character's effect didn't work before, so why would it now?"
Tony Baxter, and a few other engineers have (discreetly) implied that Constance Hatchaway was a product of Disneyland executives, not the ride designers themselves. One of the Hatbox Ghost creators has stated that now that "Hattie" is back in the ride, his next goal is to try and get the bride back to a form that's closer to her original incarnation.
The bride who came before Constance was often called "Emily" by fans.
The Hatbox Ghost tends to be called "Hattie" by fans and some Imagineers.
No Export for You: None of the Mansions are identical. Tokyo never got the Constance update to the Attic or the Floating Leota effect, while Disneyland never got the update of the Constance effect that WDW got. Walt Disney World also has the Music Room, Library, and Endless Staircase, scenes not in the Disneyland Mansion, and Tokyo has the former two, and Phantom Manor the second. Conversely, Disneyland is the only attraction to have gotten the Hatbox Ghost 2.0.
The Other Marty: In Paris's Phantom Manor, the Phantom was originally voiced by Vincent Price, but legal agreements that required the narration to be primarily in French meant that a French actor, Gérard Chevalier, was brought in to record a French version of Price's narration, which was only used for the first few months of the attraction's operation. Chevalier used to be a dub for Price in many of his movies. The only bit of Price's narration that was kept was the Phantom's evil laughter. For the 2019 Re-Haunting, Price's English narration was restored, with dialogue alternating between him and French sound-alike Bernarde Alane.
The Other Darrin: The Holiday version and some spinoff material uses Corey Burton or Joe Leahy for the Ghost Host's voice, along with Susan Blakeslee taking on Leota's voice. Kim Irvine, Toombs' daughter, provided the face for Madame Leota for the Holiday version, since she resembled her mother.
Permanent Placeholder: Imagineer Leota Ann Thomas (Leota Toombs at the time — not kidding) was originally supposed to be a stand-in for the crystal ball effect. Though Eleanor Audley's voice was used as planned, the Imagineers liked Leota's performance so much that they ended up using it, and the character was given the name Madame Leota. If you're wondering what Leota sounded like, that's her voice asking you to "Hurry back" at the end, (although this was a forced breathy voice). The fans and Disney alike call that ghost "Little Leota" as a result. (Officially, it's "the Ghostess".)
Captain Nemo's organ, from 20,000 Leagues Under the Sea, is in the Disneyland version's ballroom. (It is the only organ to be genuine; the WDW and Tokyo versions are non-functional copies.)
To save money, Imagineers tend to reuse the same animatronic molds for heads across numerous rides. They then use costumes and wigs in an effort to hide this, to mixed results. Within the Haunted Mansions:
The mold for Hitchhiking Ghost Ezra's head was used for pop-up ghosts, portraits in the Corridor of Doors, and the original Hatbox Ghost, as well as the train ticketmaster in Phantom Manor.
Many of the Disneyland version's animatronics reused the molds used for animatronics in Pirates of the Carribean.
The Mayor of Phantom Canyon in Phantom Manor uses the same mold as Dreamfinder from the original version of Epcot's Journey into Imagination with Figment.
Serendipity Writes the Plot: The iconic "stretching room" was devised because the Imagineers needed a way to get guests from the mansion that forms the facade of the building to the warehouse where the actual ride track is built, primarily because said warehouse is on the other side of the Disneyland Railroad. In Disneyland and Disneyland Paris, it's not your imagination. The room is actually stretching, because the floor you're standing on is actually an elevator and the walls stay still. Florida and Tokyo keep the effect for consistency, but simply raise the ceiling to accomplish it.
It was originally supposed to open in 1963, but the project's development was delayed by the 1964-65 New York World's Fair, as well as by the death of Walt Disney. The Haunted Mansion didn't open until 1969.
After 46 years, the Imagineers finally had the technology needed to bring the legendary Hatbox Ghost back into the ride.
The Hitchhiking Ghosts were tossed in at the last second.
A park visitor caused a piece of the glass separating the Doombuggies from the ballroom scene to crack. However, what would have been fairly expensive and awkward repair job was allowed to remain because the crack just happened to look very much like a spider's web, so a fake spider was added on top of the crack.
The ghosts dancing in the ballroom in the Anaheim version are animatronics reflected through clear glass, which resulted in the ladies leading the dance. Although this would be easy to correct, it was left in as part of the gag.
Updated Re-release: The ride has been redone several times with effects that don't break down nearly as often.
The Walt Disney World and Tokyo Disneyland versions of the ride contain additional scenes not present in the Disneyland original. Phantom Manor at Disneyland Paris takes the loose bits of story present in the original ride many steps further to produce a full history for the house, and also for Big Thunder Mountain, meaning the signature Frontierland roller coaster gets tied to the history of another ride, getting Darker and Edgier in doing so.
What Could Have Been: The Haunted Mansion went through several different ideas. The following concepts are only a very small fraction of what we know the Imagineers considered:
Until pretty late in the game, it was going to be a walkthrough instead of a ride. Some of the creators, such as Rolly Crump, still regret they had to make it a ride to this day (the reasons for the change were not artistic, but practical: it was matter of managing the crowd).
The entrance to Walt Disney World's version of the attraction was originally going to be much more elaborate◊, with guests entering through the front door and using the stretching rooms as elevators (as they do in the Disneyland version). Since there was more space in WDW, there was no need for elevators, since the show building could be on the same level aboveground. As such, in WDW, the ceilings simply rise and there's no slowly descending floor to go with them.
One proposed idea for Disneyland's original ride was to have the trees in the cemetery scene move around, and their branches reach out towards riders. However, the engineers back then couldn't figure out exactly how to get the trees to move like that, and in the years since they've never tried to re-attempt it; so the trees in the cemetery just stand perfectly still. The trees still have faces carved into them, in a manner similar to Snow White's Scary Adventures.
The Haunted Mansion Holiday overlay was planned to be added to Walt Disney World's version of the attraction; but for reasons unknown, they turned it down and the already-built decorations were sent to Tokyo Disneyland for them to do the overlay. It's been suggested that the overlay never made it to WDW because of the higher percentage of foreign and out-of-state guests at Orlando, who wouldn't be able to see the classic ride if they arrived during the holiday period. Compared to Tokyo and Anaheim, parks that cater more to local guests than to out of state visitors.
In 1965, as a response to Walt's desire to bring ghosts from all over the world into the Haunted Mansion, a Japanese television studio presented Disney with the head of Oiwa, the ghost from classical kabuki play "Yotsuya Kaidan" and the archetypical Stringy-Haired Ghost Girl. While this never came to pass, the ghost head has since found a home in the Disney Archives.
Word of Dante/Word of Saint Paul: The backstory (or rather backstories) of the Mansion come from various sources, and it has long been debated in the fandom whether any of these examples is Word of Dante or of Saint Paul:
A lot of the so-called backstory (most notably the Ghost Gallery mythos) comes from Cast Members (that is, the employees costumed and roleplaying as servants who manage the guests during the walkthrough part of the ride).
Inversely, some say that only the current incarnation of the ride is truly canon and what the original Imagineers intended in the 60's is only Word of Saint Paul.
The movies and comics were licensed by Disney legally speaking but never personally approved by the Imagineers themselves (either the old or new ones), resulting in concepts and backstories for them being of shaky canonicity.
Many Haunted Mansion fans speculate that the image of the "Hanged Man" actually represents the "Ghost Host," whom guests hanging from a noose in the Stretching Gallery. While this is not directly spelled out by the Imagineers, this is certainly a likely bit of conjecture.
Both main women in the Mansion are portrayed by two performers- with each having a voice actress separate from the filmed actress- Leota was the first, and Constance, who is achieved with a similar effect, follows suit.
Breakaway Pop Hit: Subverted with "Iz U" by Nelly, as that song underperformed on the music charts.
Development Hell: Development of a movie based on the Haunted Mansion began as early as 1990. A script was written, but the failure of the 1993 film Hocus Pocus at the box office caused Disney to redevelop the project as a made-for-TV film. However, when another made-for-TV film based on an attraction,Tower of Terror, failed in the ratings, work on The Haunted Mansion movie was pulled. It wasn't until the early 2000's, when then Disney CEO Micheal Eisner was interested in turning Disney Park attractions into feature films, that a Haunted Mansion film finally got the green light.
This is also the upcoming Guillermo del Toro take of The Haunted Mansion, where he wishes to make the film back to it's horror roots and even have an appearance of the famous Hatbox Ghost. Unfortunately, Del Toro, being Del Toro, hasn't gotten around to making it and it's unknown if the project is either in development or dead. The 2020 announcement of a new Haunted Mansion film project from Katie Dippold doesn't seem to be promising news for the Del Toro version.
Missing Trailer Scene: Early trailers and TV spots had Eddie Murphy saying "Wait until I leave, then make the dark forces come out!" in a very stern tone. In the film, while this line does come up, he says it much differently, in a more panicked, frightened tone.
In an earlier draft of the script, the Groundskeeper was a character and was going to be played by Don Knotts.
Dummied Out: Several: the Ballroom dancers both have lines recorded, but they cannot be interacted with.
Easter Egg: Upon getting the second Soul Gem from the Organist in the Ballroom, if the player looks up and towards the side opposite the windows, they can see two of the developers ride by in a Doombuggy much like in the ride.
Just like in the queue for the ride, the tombs in the Mausoleum feature the names of the developers.
Many of the textures that appear in the game are taken from the ride. Yes this includes portraits.
One painting that appears in the gallery appears to be a burning house when the frame is red. When the frame is white for that same painting, it is the façade for the Walt Disney World and Tokyo versions of the ride.
The ambience for the Attic contains a random Evil Laugh, which is a reference to the Pop-up Ghosts that once populated the area.
What Could Have Been: There was going to be a version released on the Game Boy Advance, but despite being complete, it was blocked from released by Take 2.
Dummied Out audio files within the game reveal that Leota would be able to cast spells that would cause Zeke to cheat death or stay invincible for a period of time. Another line suggests that Zeke would have to escape from the Reaper after being scared to death (ie lose all HP) to reclaim his body. The music for this also remained on the disc as an "Action" track (other action tracks include the Attic / Sun Room and the Pantry / Oservatory).
Other failed animated adaptations included a Halloween Special produced by illustrator Gris Grimly that was planned for the 45th anniversary and a direct to video film planned in the late 90s that was canned in favor of the 2003 live-action film.